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December 31, 2006

Nerves from the brain

From: Malaysia Star - Malaysia - Dec 31, 2006

The cranial nerves are twelve pairs of nerve tissue that arise from the brain itself, rather than the spinal cord. They are essential for many of the things that we take for granted, such as smelling, seeing and eating.

MY friend mentioned that she was diagnosed with several brain tumours impinging on her brainstem. These tumours, she said, are likely to impinge upon her cranial nerves. What are cranial nerves?

There are twelve cranial nerves altogether. These are twelve pairs of nerve tissue that arise from the brain itself, rather than the spinal cord. In order to innervate their target organs, they must exit/enter the cranium through openings in the skull (cranium). Hence, they are called cranial nerves.

The nerves that arise from the spinal cord are called spinal nerves.

What do the cranial nerves do?

They play a very important role indeed. The first cranial nerve is the olfactory nerve. It is an entirely sensory nerve. There are specialised sensory receptive parts of the olfactory nerve which are located in the olfactory mucosa (a layer) of the upper parts of your nose.

During breathing, air molecules attach to the olfactory mucosa and stimulate the olfactory receptors. Olfactory bulb cells then transmit electrical activity to other parts of the central nervous system via the olfactory tract. The smell that you are trying to identify is then interpreted by the brain.

That’s why when you have a cold, you lose your sense of smell.

The second cranial nerve is the optic nerve.

The optic nerve starts from the cells of your retina inside your eye, where images that you see from the outside world are projected upside down. The cells then transmit the signals to your brain via the optic nerve.

The optic nerve and tract has quite a complicated pathway. Anything that happens to this pathway (such as a tumour pressing the optic chiasm where the nerve fibres cross) will result in a certain part of an image (such as a vertical half of an image from one eye or the other, or even both) deleted from your vision.

The brain will then interpret what you are seeing. The part of the brain that does this is the occipital lobe, the back portion.

I have heard that the 3rd cranial nerve has something to do with movements of the eye.

Yes. Vision itself is transmitted through the 2nd cranial nerve, but the eye also has to move up and down and sideways. The 3rd cranial nerve is called the oculomotor nerve.

This nerve is in charge of supplying the levator palpebrae superioris, which lifts your upper eyelid. It also supplies the superior rectus (moving your eye upward and inward, as in trying to look towards the inner contours of your eyebrow), medial rectus (moving your eye inward towards the tip of your nose), inferior rectus (downward and inward) and inferior oblique (moving your eye upward and outward) muscles of the eye.

The 3rd cranial nerve also has an autonomic component that constricts your pupils.

A patient with a 3rd nerve palsy will then fail to raise his upper eyelid (ptosis), fail in all the movements of the eye described above and have a dilated pupil in the affected eye.

The 4th cranial nerve (trochlear nerve) innervates the superior oblique muscle of the eye, which is responsible for turning the eye outwards and downward.

The 6th cranial nerve (abducens nerve) is responsible for the lateral rectus muscle, which turns the eye outward. Naturally, a person with this palsy has a failure to look to his affected side.

What about the 5th and 7th cranial nerves? I heard they are huge.

Yes. The 5th (trigeminal) nerve is composed of three large branches – ophthalmic (V1), maxillary (V2) and mandibular (V3) branches.

The ophthalmic branch is responsible for the sensation of the cornea and the area of skin from your forehead to your eyelids. The maxillary division gives sensation to the part of your face over the maxilla, nose, teeth of your upper jaw and palate. The mandibular branch takes care of your cheek, jaw, side of head and whether your first 2/3 of your tongue feels any pain/hot/cold.

The mandibular branch is also responsible for the muscles of chewing.

The 7th (facial nerve) is a very long nerve and partial to injury. It supplies the muscles for facial expression – furrowing your brow, closing your eye, smiling, puffing out your cheeks. It also supplies the taste component for the first 2/3rds of your tongue.

If any part of this nerve is affected, you won’t be able to close your eye on the affected side, furrow your brow or smile.

Which is the nerve responsible for hearing?

The 8th (vestibulocochlear) nerve. True to its name, it has two main branches. The vestibular part is responsible for the position and movement of the head. The cochlear part is for hearing.

Next we come to the nerves arising from the brainstem.

The 9th (glossopharyngeal) nerve assists in swallowing. It also supplies the sensation and taste for the last 1/3 (back) part of your tongue.

The 10th (vagus) nerve is the largest nerve in the body. It supplies your pharynx (in charge of constricting your throat), your larynx (voice box), the involuntary musculature of your bronchi inside your lungs, your heart, your intestinal tract, your liver and pancreas.

The 11th (accessory) nerve supplies the muscles of your soft palate, pharynx and larynx and the great neck muscles.

The 12th (hypoglossal) nerve supplies the muscles that move your tongue.

To help you remember the nerves in their order, medical students use this mnemonic (the first alphabet of each word correlates with the first alphabet of each nerve): Old Opie Occasionally Tries Trigonometry And Feels Very Gloomy, Vague and Hypoactive.

Dr YLM graduated as a medical doctor, and has been writing for many years on various subjects such as medicine, health advice, computers and entertainment. The information contained in this column is for general educational purposes only. Neither The Star nor the author gives any warranty on accuracy, completeness, functionality, usefulness or other assurances as to such information. The Star and the author disclaim all responsibility for any losses, damage to property or personal injury suffered directly or indirectly from reliance on such information.

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